Following the 1997 creation of the first laser to emit pulsed beams of atoms, MIT researchers report in the May 16 online version of Science that they have now made a continuous source of coherent atoms. This work paves the way for a laser that emits a continuous stream of atoms.
MIT physicists led by physics professor Wolfgang Ketterle (who shared the 2001 Nobel prize in physics) created the first atom laser. A long-sought goal in physics, the atom laser emitted atoms, similar in concept to the way an optical laser emits light.
"I am amazed at the rapid progress in the field," Ketterle said. "A continuous source of Bose-Einstein condensate is just one of many recent advances."
Because the atom laser operates in an ultra-high vacuum, it may never be as ubiquitous as optical lasers. But, like its predecessor, the pulsed atom laser, a continuous-stream atom laser may someday be used for a variety of applications in fundamental physics.
It could be used to directly deposit atoms onto computer chips, and improve the precision and accuracy of atomic clocks and gyroscopes. It could aid in precision measurements of fundamental constants, atom optics and interferometry.
A continuous stream laser could do all of these things better than a pulsed atomic laser, said co-author Ananth P. Chikkatur, a graduate student in physics at MIT. "Similar to the optical laser revolution, a continuous-stream atom laser might be useful for more things than a pulsed laser," he said.
In addition to Ketterle and Chikkatur, authors include graduate students Yong-Il Shin and Aaron E. Leanhardt; David F. Kielpinski, a postdoctoral fellow in the Research Laboratory of Electronics (RLE); Edem Tsikata, a senior majoring in physics; MIT-affiliate Todd L. Gustavson; and David E. Pritchard, the Cecil and Ida Green Professor of Physics and a member of the MIT-Harvard Center for Ultracold Atoms and the RLE.
NEW FORM OF MATTER
An important step toward the first atom laser was the creation of a new form of matter--the Bose-Einstein condensate (BEC). BEC forms at temperatures around one millionth of a degree Kelvin, a million times colder than interstellar space.
Ketterle's group had developed novel cooling techniques that were key to the observation of BEC in 1995, first by a group at the University of Colorado at Boulder, then a few months later by Ketterle at MIT. It was for this achievement that researchers from both institutions were honored with the Nobel prize last year.
Ketterle and his research team managed to merge a bunch of atoms into what he calls a single matter-wave, then use fluctuating magnetic fields to shape the matter-wave into a beam much like a laser.
To test the coherence of BEC, the researchers generated two separate matter-waves, made them overlap and photographed a so-called "interference pattern" that only can be created by coherent waves. The researchers then had proof that they had created the first atom laser.
Since 1995, all atom lasers and BEC have been produced in a pulsed manner, emitting individual pulses of atoms several times per minute. Until now, little progress has been made toward a continuous BEC source.
While it took about six months to create a continuous optical laser after the first pulsed optical laser was produced in 1960, the much more technically challenging continuous source of coherent atoms has taken seven years since Ketterle and colleagues first observed BEC in 1995.
A NEW CHALLENGE
Creating a continuous BEC source involved three steps: building a chamber where the condensate could be stored in an optical trap, moving the fresh condensate and merging the new condensate with the existing condensate stored in the optical trap. (The same researchers first developed an optical trap for BECs in 1998.)
The researchers built an apparatus containing two vacuum chambers: a production chamber where the condensate is produced and a "science chamber" around 30 centimeters away, where the condensate is stored.
The condensate in the science chamber had to be protected from laser light, which was necessary to produce a fresh condensate, and also from hot atoms. This required great precision, because a single laser-cooled atom has enough energy to knock thousands of atoms out of the condensate. In addition, they used an optical trap as the reservoir trap, which is insensitive to the magnetic fields used for cooling atoms into a BEC.
The researchers also needed to figure out how to move the fresh condensate--chilled to astronomically low temperatures--from the production chamber to the science chamber without heating them up. This was accomplished using optical tweezers--a focused laser light beam that traps the condensate.
Finally, to merge the new condensate with the existing condensate in the science chamber, they moved the new condensate in the tweezers into the science chamber by merging the condensates together.
A BUCKET OF ATOMS
If the pulsed atom laser is like a faucet that drips, Chikkatur says the new innovations create a sort of bucket that collects the drips without wasting or changing the condensate too dramatically by heating it. This way, a reservoir of condensate is always on hand to replenish an atom laser.
The condensate pulses are like a dripping faucet, where the drops are analogous to the pulsed BEC production. "We have now implemented a bucket (our reservoir trap), where we collect these drips to have a continuous source of water (BEC)," Chikkatur said. "Although we did not demonstrate this, if we poke a hole in this bucket, we will have a steady stream of water. This hole would be an outcoupling technique from which we can produce a continuous atom laser output.
"The big achievement here is that we have invented the bucket, which can store atoms continuously and also makes sure that the drips of water do not cause a lot of splashing (heating of BECs)," he said.
The next step would be to improve the number of atoms in the source, perhaps by implementing a large-volume optical trap. Another important step would be to demonstrate a phase-coherent condensate merger using a matter-wave amplification technique pioneered by the MIT group and a group in Japan, he said.
This work is funded by the National Science Foundation, the Office of Naval Research, the Army Research Office, the Packard Foundation and NASA.
A version of this article appeared in MIT Tech Talk on May 22, 2002.